The present invention relates to fluid conducting bellows, expansion joints and flexible couplings of the type having spaced apart pipe flanges and a bellows element interconnecting the flanges.
While the present invention has utility in combination with a metal bellows element, its principal application is to bellows elements formed from plastic and/or elastomeric material, and is especially useful with elements formed from polytetrafluoroethylene (PTFE) resin. For simplification of discussion, the expression "bellows assembly" shall be used to designate generically any of the fluid conducting or piping components variously referred to as bellows, expansion joints and flexible couplings.
Bellows assemblies are used in piping systems for various isolation functions, often in association with a pump to isolate the vibration of the latter from the input and output lines. Assemblies having a PTFE bellows element are generally used in piping systems handling caustic or corrosive fluids for which PTFE resin has unique tolerance. However, such use makes it extremely important that gross failure of the bellows element be prevented, or if it cannot be prevented, that some steps be taken to protect personnel and property that might be in the vicinity of a rupturing bellows element.
As a means for minimizing the possibility of bellows failure in use, careful attention has been paid in the past to the use of extreme conservatism in specifying the permissible installation and operating parameters for a given bellows assembly. With flange mounted bellows assemblies it has been customary practice to provide factory installed limit bolts that interconnect the flanges of the bellows assembly and that are secured by lock nuts or the like. In spite of manufacturer admonition to the contrary, installers have been known to remove the limit bolts and subject the bellows assembly to excessive strain in order to bolt it into the plumbing system.
Besides deliberate alteration of the as-manufactured bellows assembly during installation, bellows assemblies are frequently damaged through carelessness or sheer accident between point of manufacture and completion of installation. For example, workers have been known to drop tools on the assemblies or drop the assembly itself where merely a nick in the wrong place can give rise to potential failure.
Aside from faulty installation, there is ever present, for one reason or another, the potential for bellows element failure. For example, there might occur a sudden pressure surge in the line, or the temperature of the fluid might exceed safe limits for the material of which the bellows element is made.
History in the industry has demonstrated that the fears of catastrophic failure are not unfounded. There have occurred numerous failures that have resulted in serious injury to operating personnel.
Thus, the users of such bellows assemblies have resorted to various expedients in an endeavor to improve safety. One expedient takes the form of a piece of fabric that can be wrapped around an installed bellows assembly and is provided with draw strings in opposing hemmed edges for closing the fabric piece at the ends down over the flanges onto the associated pipe sections. The concept assumes that if the bellows element springs a leak or ruptures, the fabric wrapping will prevent fluid from being sprayed around with the possibility of causing injury. Such solution to a significant problem is, at best, only in the nature of a stop-gap measure. Moreover, the fabric wrapping is not a seal and it is not fully effective when a major failure occurs in the underlying bellows. In addition, the fabric wrapping technique involves covering flanges and bolts and tends to trap vapors which are often corrosive and induce corrosion in the bolts. But since the bolts are covered they are not readily inspected. This problem is of sufficient significance that in some establishments the use of fabric wrapping is actually prohibited.
In the copending application of Irving D. Press. for "A Fail-Safe Bellows Assembly", Ser. No. 07/100,467, filed Sept. 24, 1987, there is described and claimed an arrangement that eliminates the usual limit bolts and replaces them with a cylindrical guard that encircles the bellows element with its associated coupling flanges and interconnects said flanges so as to permit a limited safe magnitude of elongation and articulation. The guard is connected to one of said flanges, with a coupling or linking to the other flange that permits the desired safe freedom of movement. In one embodiment one of the flanges has the guard clamped to its periphery while the other flange couples to the guard with pins and apertures. Another embodiment uses pins projecting radially inwardly from the guard at one end, replacing the pins in the flange and cooperating apertures in the guard.
In addition to the guards, an elastic sleeve is joined to the periphery of each flange with a fluid-tight seal, and a vent passage through one flange interconnects the inter-flange space with the exterior for safe discharge of any fluid escaping from the bellows element.
However, all of the embodiments described in the aforesaid application require either flanges of unequal diameter or a guard cylinder that has a larger diameter at one end than at the other end with a transition therebetween. While the described assemblies are capable of carrying out the desired objective of safety, the construction has been found to be unnecessarily costly and to have other disadvantages.
It is, therefore, an object of the present invention to provide a true fail-safe bellows assembly that is superior to those described in the copending application. As with the earlier application, it is an object to provide a bellows assembly whose safety provisions cannot be defeated by the user and which, if the bellows element should fail, whether it be a pin-hole breach or a catastrophic rupture, will safely carry off the escaping fluid in a rigorously controlled manner. At the same time, it is an object of the invention to provide an indication of a leak at its initial stages when a catastrophic failure of the bellows element has not occurred. It is also an object of the invention to provide enhanced freedom of the bellows element from damage by external means.
With the prior embodiments it was discovered that the secondary seal, upon failure of the bellows element, can, under certain circumstances, be subject to an initial pressure surge sufficient to blow out the seal through the slots in the cylindrical guard or through the clearance gap between flange and guard. While fabric reinforcement of the secondary seal could strengthen the seal sufficiently to prevent blow out, such reinforcement can, under certain circumstances, restrict elastic elongation creating additional problems. Therefore, it is a further object of the present invention to provide a fail-safe bellows assembly in which the secondary seal does not interfere with articulation of the principal bellows element and, yet, is prevented from blowing out under bellows-failure precipitated pressure surges in the inter-flange space.
Another object of the present invention is to provide the desired safeguards in a more economical manner.